Known in the art is an internal combustion engine which is provided with a transmission mechanism which is comprised of a hollow sleeve which can rotate about an axis and a control rod which engages with a female thread formed inside of the hollow sleeve and is made to move in an axial direction of the hollow sleeve if the hollow sleeve is made to rotate, wherein the control rod is connected to a link mechanism which controls a stroke of a piston, an electric motor is used to make the hollow sleeve rotate and thereby make the control rod move, and thereby the stroke of the piston is made to change, that is, the mechanical compression ratio is made to change (for example, see Patent Literature 1).
In this internal combustion engine, the transmission mechanism is formed into an irreversible type so that even if the compression pressure or the combustion pressure changes, the piston stroke will not change, that is, the control rod will not move.